The present invention relates to electronic communication systems, and more particularly to a system and method for adapting parameters of radio links to accommodate changes in the environment of the communication system.
Wireless communication systems transmit communication signals on one or more carrier waves. Many existing radio communication systems use Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) channel access techniques. In FDMA access systems, a channel may be defined by one or more radio frequency bands within a given frequency spectrum into which a communication signal's transmission power is concentrated. Interference in FDMA systems may be caused by signals transmitted on adjacent channels (adjacent channel interference) and signals transmitted on the same channel (co-channel interference). Interference from adjacent channels may be limited by the use of band-pass filters that filter out energy outside the specified frequency band.
In TDMA access systems a channel comprises a time slot in a periodic train of time slots of a carrier wave having a given frequency. A given signal's energy is confined to one or more of the designated time slots. These time slots may be organized into groups commonly referred to as frames. Adjacent channel interference may be limited by the use of a time gate or other synchronization element that only passes signal energy received at the proper time. In TDMA access systems, capacity is limited by the available time slots and by limitations imposed by channel reuse.
In Code Division Multiple Access (CDMA) systems, a communication channel is defined by a digital code. In a direct sequence-CDMA (DS-CDMA) spread spectrum transmitter, for example, a digital symbol stream for a given dedicated or common channel at a basic symbol rate is spread to a chip rate. This spreading operation involves applying a channel-unique spreading code, sometimes referred to as a signature sequence, to the symbol stream that increases its rate (bandwidth) and introduces redundancy. The intermediate signal comprising the resulting data sequences (chips) may be added to other similarly processed (i.e., spread) intermediate signals relating to other channels. A base station-unique scrambling code (often referred to as the “long code” since it is in most cases longer than the spreading code) is then applied to the summed intermediate signals to generate an output signal for multi-channel transmission over a communication medium. Multiple intermediate signals may overlap in both the frequency domain and the time domain. A receiver recovers its intermediate signal by correlating the received signal with the appropriate scrambling and spreading codes to despread, or remove the coding from the desired transmitted signal and return to the basic symbol rate. Where the spreading code is applied to other transmitted and received intermediate signals, however, only noise is produced.
Digital communication systems use a variety of linear and non-linear modulation schemes to communicate voice or data information in bursts. These modulation schemes include GMSK, Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. GMSK modulation scheme is a non-linear low-level modulation (LLM) scheme with a symbol rate that supports a specified user bit rate. High-level modulation (HLM) schemes can be used to increase user bit rates. Linear modulation schemes, such as QAM schemes, may have different levels of modulation. For example, 16 QAM scheme is used to represent the sixteen variations of 4 bits of data. On the other hand, a QPSK modulation scheme is used to represent the four variations of 2 bits of data.
In addition to various modulation schemes, digital communication systems can support various channel coding schemes used to increase communication reliability. Channel coding schemes code and interleave data bits of a burst or a sequence of bursts to prevent their loss under degraded RF link conditions, for example, when RF links are exposed to fading. In general, increasing the number of coding bits increases the bit error detection and correction capabilities, but reduces the user bit rate, since coding bits reduce the number of user data bits that can be transmitted in a burst.
Increases in wireless communication has generated a need for additional voice and data channels in cellular telecommunication systems. To accommodate this need, operators of wireless networks have increased the number of base stations in operation. Increasing the number of base stations has reduced the distance between base stations, which creates increased interference between mobile stations operating on the same frequency in neighboring or closely spaced cells.
Link adaptation techniques may be invoked to accommodate increased interference on a communication link. Link adaptation techniques provide the ability to change a communication link protocol, which may be defined by a combination of modulation scheme, channel coding (e.g., FEC coding), and/or the number of used time slots. Dynamic link adaptation methods permit the link protocol to be changed in response to changing channel conditions. Generally, link adaptation methods adapt a system's link protocol to achieve desired performance over a broad range of interference conditions. Exemplary link adaptation schemes are described in U.S. Pat. Nos. 5,574,974; 5,898,928; 6,122,293; 6,134,230; and 6,167,031, which are incorporated by reference herein.
Recently, a radio interface referred to as Bluetooth was introduced to provide wireless, ad hoc networking between mobile phones, laptop computers, headsets, PDAs, and other electronic devices. Some of the implementation details of Bluetooth are disclosed in this application, while a detailed description of the Bluetooth system can be found in “BLUETOOTH—The universal radio interface for ad hoc, wireless connectivity,” by J. C. Haartsen, Ericsson Review No. 3, 1998. Further information about the Bluetooth interface is available on the Official Bluetooth Website on the World Wide Web at http://www.bluetooth.org.
Radio communication systems for personal use differ significantly from radio systems like the public mobile phone network. Public mobile phone networks use a licensed band which is fully controlled by the network operator and provides a substantially interference-free channel. By contrast, personal radio communication equipment operates in an unlicensed spectral band and must contend with uncontrolled interference. One such band is the globally-available ISM (Industrial, Scientific, and Medical) band at 2.45 GHz. The band provides 83.5 MHz of radio spectrum. Since the ISM band is open to anyone, radio systems operating in this band must cope with unpredictable sources of interference, such as baby monitors, garage door openers, cordless phones, and microwave ovens. Interference can be reduced using an adaptive scheme that seeks out an unused part of the spectrum. Alternatively, interference can be suppressed by means of spectrum spreading. In the U.S., radios operating in the 2.45 GHz ISM band are required to apply spectrum-spreading techniques if their transmitted power levels exceed about 0 dBm.
Bluetooth radios use a frequency-hop/time-division-duplex (FH/TDD), spread spectrum channel access scheme. In the United States and in most European countries, Bluetooth radios utilize 79 RF channels spaced 1 MHz apart in the 83.5 MHz ISM band. During a connection, radio transceivers “hop” from one frequency band to another in a pseudo-random fashion. The frequency hopping sequence is determined by the device address of a Bluetooth unit. The time dimension is divided into slots of 625 μs, resulting in a nominal hop rate of 1600 hops/second. Further, slots are used alternately for transmitting and receiving, resulting in a TDD scheme. These features allow for low-cost, low-power, narrowband transceivers with strong immunity to interference.
Generally, the performance of a communication channel is a function of the ratio S/(N+I), where S is the received signal, I is the interference, and N the noise. For radio channels, S is a function of the transmit power and propagation loss on the communication path. Since radio signals propagate omni-directionally, the signal strength declines as function of the distance from the transmitter. Also, the signal may be attenuated by objects blocking the communication path between the transmitter and receiver. In mobile communication systems each of these variables may change over time. The noise N includes thermal noise present in space and thermal noise generated in the electronic circuitry of the receiver. Noise N is normally determined by the bandwidth of the channel and the quality of the receiver, and may vary as function of temperature. The interference I is generated by other radio transmitters in the area and also may vary over time. The interference I can be divided into three components: a co-channel component representing external interference that falls within the channel bandwidth, an adjacent-channel component representing external interference that falls outside the channel bandwidth, and “self-interference” representing interference created by the signal S itself and caused by distortion of the channel.
Link adaptation modifies link parameters to ensure the ratio S/(N+I) remains above an acceptable threshold. In conventional cellular systems, channel planning techniques may be used to reduce interference I from users in the same geographical area. The remaining S/N then determines the link performance. Degradation of the S/N ratio can be reduced by modifying S, for example by implementing suitable power control routines. Public communication systems compatible with the European GSM standard perform this type of link adaptation.
Existing link adaptation techniques were developed for coordinated radio communication systems, in which cell sizes may be adjusted and channel reuse schemes may be implemented to ensure that co-channel interference levels and adjacent channel interference levels are maintained below a maximum level. Because uncoordinated radio systems are unable to control interference levels, the effectiveness of existing link adaptation techniques is limited in uncoordinated radio systems. For example, in an uncoordinated radio system, an interfering transmitter may be much closer to the receiver than the intended transmitter or the transmit power of the interfering transmitter may be much larger than the transmit power of the intended transmitter. In either case, the received signal level may be similar to or smaller than the received interference level. This is usually referred to as the near-far problem. Link adaptation schemes based on changing the coding rate or changing the modulation scheme may be inadequate to address interference caused by the near-far problem. Also, existing link adaptation schemes may affect the net user rate. For example, the channel bandwidth in a GSM system is constant. Increasing the amount of FEC coding or implementing a more robust modulation scheme typically decreases the net user rate.
Accordingly, there remains a need in the art for link adaptation techniques useful in radio systems which incur relatively high interference levels, like those incurred in uncoordinated radio systems. Further, there is a need for link adaptation techniques that attempt to maintain a substantially constant net user rate and bit-error-rate on the communication channel under changing signal and interference conditions.